Anemones won't "poison" the tank when they die. Just A LOT of excess nutrients as they just erode away.
As stated you have several issues.
Tank is way too young for one anemone let alone two. Which is probably why they died. Plus they require great water quality which you won't get with tap. Also your tank is too small for any tangs.
You probably don't have the biological filtration to handle the die off from the nems and caused an ammonia spike which has claimed the tangs and will likely kill the rest of your fish.
As stated you should probably rehome all of your livestock and do a 100% water change and wait for a new cycle to complete. See if your lfs
can hold them temporarily.
Regarding your rodi
there are a lot of vids on YouTube on how to set one up.